1. Field of the Invention
This invention relates to a process of directly reducing iron oxide-containing materials, preferably iron ores, to produce sponge iron in a rotary kiln below the softening and melting point of the charge, comprising charging solid carbonaceous reducing agents having a high content of volatile constituents into a rotary kiln at its charging end, feeding oxygen-containing gases through shell tubes at controlled rates into the free kiln space, and moving the charge and the gas atmosphere in the rotary kiln countercurrently to each other.
2. Discussion of the Prior Art
In the production of sponge iron in a rotary kiln by means of solid carbonaceous reducing agents there is a risk of a reoxidation of the sponge iron in the final portion of the reduction zone unless the charge in said final portion contains a surplus of solid reducing agent which maintains in the bed a reducing atmosphere with respect to sponge iron. For this reason the process is usually carried out with a relatively high surplus of carbon. The surplus reducing agents must be separated and should be recycled to the rotary kiln, for economical reasons. On the other hand, other components of the charge, particularly fine-grained components, often tend to deposit on the reducing agent during its travel through the kiln, owing to the large surface of the reducing agent, so that the melting point of the coal ash is often decreased below the reduction temperature and incrustations may form in the kiln. Because the surplus reducing agent or the recycled reducing agent has a low bulk density, it occupies a relatively large volume in the kiln, compared to the metallic component. As that occupied kiln volume cannot be used for the throughput of mettalic component, it tends to decrease the residence time of the metallic component. Moreover, the recirculated reducing agent must be re-heated to the reduction temperature and must be cooled in the material discharged from the kiln. This results in a longer heating-up time, a larger heat consumption and a discharge of a larger quantity of material, which must be cooled.
It has already been proposed in West German Offenlegungschrift (DOS) No. 19 42 541 to use a reducing agent consisting of highly reactive coals, which disintegrate at the kiln temperature, to blow in said coals from the discharge end, to distribute them at least throughout the length of the reduction zone, and to control the operation of the rotary kiln at such a throughput rate that the desired degree of metallization is achieved and the material discharged from the kiln contains carbon at a rate which is less than 1.5 percent referred to the amount of iron oxide-containing charge material. The surplus cannot be decreased to consist only of carbon deposited on sponge iron but for safety reasons in case of fluctuations in the operation, the material discharged from the kiln must contain additional carbon at a rate of 0.1 to 0.7 percent of the rate at which iron oxide-containing material is charged to the kiln. In order to ensure these small surplusses, the reducing agent must be distributed over the reduction zone in relation to the removal of oxygen so that coal is available just in the required quantity in any given part of the charge. Particularly in large rotary kilns such exact distribution requires a considerable expenditure for the feeders and their control means. Besides, the coal must always be supplied in the same particle size distribution. There must be no short-time fluctuations in the operation of the kiln because they would require a larger safety surplus.
It is known from West German Pat. No. 10 32 550 that the reactions taking place in the charge during the reduction with solid reducing agents can be accelerated by a blowing of air or reducing gases through the charge which has been heated to the reduction temperature by a central burner. A decribed of the required carbon surplus is not described in that publication and is not accomplished in the proccess disclosed therein.
It is known from U.S. Pat. No. 3,180,725 to charge the solid carbonaceous reducing agent into the kiln at a point which is spaced from the charging end and to add a surplus, which may be separated from the material discharged from the kiln and may be recycled. Gaseous or liquid reducing agent may be blown into the charge in the reducing zone and may be used to produce solid carbon. The required surplus is not affected by that practice.
In accordance with West German Pat. No. 12 26 126, hydrocarbons and air are introduced into the charge bed to provide for a carbon surplus in the material discharged from the kiln.
West German Pat. No. 12 73 549 describes the production of low-sulfur sponge iron in which substances that contain or release hydrogen are blown into the charge in the reduction zone. This does not affect the required carbon surplus.
U.S. Pat. No. 2,663,632 describes a two-stage reduction process, in which a metallization of 50 to 80 percent is effected by means of solid reducing agents in the first stage and the further reduction is effected in a separate furnace by means of reducing gases, which cool the sponge iron at the same time. Two units and a complicated transfer mechanism are required in that process. If the gases from the second stage are used in the first stage, only coke can be used as a solid reducing agent.
It is known from German Offenlegungschrift No. 21 46 133 and from U.S. Pat. No. 3,182,980 to reduce pellets only by means of hydrocarbons or to process pellets in which coal powder or coal dust has been incorporated to provide part of the required fuel. The hydrocarbons are fed into the charge in the reduction zone through tubular air nozzles. This feeding begins at that point of the kiln at which the charge has approximately reached the reduction temperature. Oxidizing gases are blown through the tubular air nozzles into the free kiln space. That processes lack the advantages which are afforded by the addition of solid reducing agent to the charge. As a result of such addition, the reducing agent is uniformly distributed and acts uniformly throughout the length of the rotary kiln. In addition, the charge bed is less compact and the temperature threshold for the sintering is increased. Another disadvantage resulting from the blowing of air through tubular air nozzles resides in that flames are formed at the kiln wall.
It is known from German Offenlegungschrift No. 25 01 182 to charge the rotary kiln at its charging end with agglomerated, highly reactive, solid carbonaceous reducing agents, which contain 20 to 50 percent of volatile constituents, and to heat up said reducing agents at such a rate that they substantially disintergrate to a fine-grained material within the heating-up zone of the rotary kiln before the end of said zone. In that process, the rate at which carbon must be discharged from the kiln may be less than 1 percent of the rate at which iron oxide-containing material is fed, particularly when a reducing flame is maintained in the final portion of the reduction zone by a central burner which is operated with oil or gas and a deficiency of air.
It is an object of the invention to provide a process for the production of sponge iron by means of solid carbonaceous reducing agents in which the reoxidation of the sponge iron in the final portion of the reduction zone of the rotary kiln is reliably avoided with simple means whereas a surplus of solid carbon is minimized.